H. Kusuhara et al., REDUCED FOLATE DERIVATIVES ARE ENDOGENOUS SUBSTRATES FOR CMOAT IN RATS, American journal of physiology: Gastrointestinal and liver physiology, 38(4), 1998, pp. 789-796
We examined the role of the canalicular multispecific organic anion tr
ansporter (cMOAT) in the biliary excretion of reduced folate derivativ
es in vivo and in vitro using normal [Sprague-Dawley rats (SDR)] and m
utant [Eisai hyperbilirubinemic rats (EHBR)] rats whose cMOAT is hered
itarily deficient. In vivo, the biliary excretion of endogenous tetrah
ydrofolate (H(4)PteGlu), 5-methyltetrahydrofolate (5-CH3-H(4)PteGlu),
and 5,10-methylenetetrahydrofolate (5,10-CH2-H(4)PteGlu) in EHBR was r
educed to 8.2%, 1.9%, and 5.5% of those in SDR, respectively, whereas
that of 10-formyltetrahydrofolate (10-HCO-H(4)PteGlu) was detected onl
y in SDR and not in EHBR. Bile drainage caused reduction of endogenous
plasma folate concentrations in SDR but not in EHBR. In vitro, signif
icant ATP-dependent uptake of H-3-labeled 5-CH3-H4PteGlu into canalicu
lar membrane vesicles was observed only in SDR. This ATP-dependent upt
ake was saturable with a Michaelis constant (K-m) value of 126 mu M, w
hich was comparable with its inhibitor constant (K-i) value of 121 mu
M for the ATP-dependent uptake of a typical cMOAT substrate, 2,4-dinit
rophenyl-S-glutathione (DNP-SG). Vice versa, DNP-SG inhibited the upta
ke of 5-CH3-H(4)PteGlu with a K-i of 35 mu M, which was similar to its
K-m value. In addition, H(4)PteGlu and 5,10-CH2-H(4)PteGlu also inhib
ited the ATP-dependent uptake of DNP-SG. These results indicate that 5
-CH3-H4PteGlu and other derivatives are transported via cMOAT. Therefo
re, reduced folate derivatives are the first endogenous substrates for
cMOAT that do not contain glutathione, glucuronide, or sulfate moieti
es.